An automatic transmission is configured to automatically switch shift gear stages, that is, power transmission paths by selective engagement of a plurality of hydraulic friction engaging elements, each of the power transmission paths being constituted by a planetary gear mechanism and the like. Each of the shift gear stages is basically formed by the engagement of two friction engaging elements. However, a first gear stage in a D range (drive range) is conventionally formed by one friction engaging element and a one-way clutch (OWC) for the purpose of, for example, smoothing of a gear shift operation.
However, the OWC is high in cost. In addition, the OWC incurs rotational resistance at gear stages other than the first gear stage in the D range, and this hinders improvement of fuel efficiency of an engine. On this account, discontinuation of the OWC has been proposed or executed in recent years.
For example, PTL 1 discloses an automatic transmission with six forward gear stages and one reverse gear stage, the automatic transmission including as friction engaging elements: a forward clutch (hereinafter referred to as a “low clutch”) C1 engaged at the first to fourth gear stages; a high clutch C2 engaged at the fourth to sixth gear stages; a 3 & 5 & reverse brake (hereinafter referred to as an “R35 brake”) B1 engaged at the third, fifth, and reverse gear stages; a 2 & 6 brake (hereinafter referred to as a “26 brake”) B2 engaged at the second and sixth gear stages; and a low reverse brake (hereinafter referred to as an “LR brake”) B3 engaged at the first and the reverse gear stages. Instead of the OWC, the LR brake B3 is engaged at the first gear stage in the D range. The first gear stage is formed by the engagement of the LR brake B3 and the low clutch C1.
A hydraulic control circuit of the automatic transmission includes electronic hydraulic control valves corresponding to the respective friction engaging elements. The electronic hydraulic control valves control supply of the oil pressure to the five friction engaging elements and discharge of the oil pressure from the five friction engaging elements. The hydraulic control circuit controls the operations of the hydraulic control valves to form the shift gear stage corresponding to a driving state.